System and method for radio transmitter acquisition

ABSTRACT

A method of a receiver determining the timing of a signal transmitted in a time-slotted manner, the signal comprising a sequence of information which is repeated at a known interval and has at least a known minimum length. The method performs correlation operations between groups of received slots of information, the groups spaced by the known interval. The groups are moved through the received signal, adding and removing slots, to locate a maximum correlation value sum for the group which should correspond to the timing of the slot. The method also can be used to determine a frequency offset at the receiver and/or an initial phase.

FIELD OF THE INVENTION

[0001] The present invention relates to system and method for a receiverto acquire a transmitter of a radio signal. More specifically, thepresent invention relates to a system and method for a radio receiver toacquire and interpret a multiplexed radio signal from a transmitterwhich employs a slotted, or other time-based, transmission structure.

BACKGROUND OF THE INVENTION

[0002] Advanced radio communications systems are being developed anddeployed to provide wireless voice and data services. One example ofsuch an advanced radio communications system presently being developedis that specified by the Third Generation Partnership Project, or 3GPP,which is an international partnership of telecommunications standardsorganizations. More information about the communications systems beingspecified by the 3GPP, including technical specifications, can be foundon their web page www.3gpp.org, or from the various memberorganizations.

[0003] The proposed 3GPP system is a communications system which employscellular-type networks to permit communications between fixed basestation transceivers and customer transceivers which can be mobile orfixed. One of tasks such an advanced communications system must performis the acquisition of a base station transmitter by the receiver in acustomer device transceiver when the customer device is powered on andat various other times, for example to support handoff of the customerdevice between base stations. Acquisition of the base stationtransmitter by the customer device receiver includes many of the stepsrequired for communication to commence between the base station and thecustomer device, including determining synchronization/timing, carrieroffset and the initial phase of signals received by the customer devicefrom the base station.

[0004] Due to the complexity of the structure and arrangement of thephysical communication channels, multipath effects, etc., acquisitioncan be difficult and/or computationally expensive to achieve. As will beapparent to those of skill in the art, this difficulty and/or complexitycan increase the cost of customer devices and/or can result in poorservice, for example if acquisition requires too long a time to achieve.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide a novelmethod and system for a radio receiver to acquire a radio transmitter ina communication system, the method and/or system obviating or mitigatingat least some of the above-identified disadvantages of the prior art.

[0006] According to a first aspect of the present invention, there isprovided a method for a radio receiver to acquire timing information fora radio transmitter which transmits, in a time-slotted arrangement, atleast one signal which includes a sequence of information indicating theslot timing and having a length at least equal to a known minimum lengthand which sequence is repeated in said at least one signal at a knowninterval, the method comprising the steps of:

[0007] (i) receiving a number of slots of said at least one signal whichis at least sufficient to allow reception of two repetitions of saidknown minimum length of said sequence;

[0008] (ii) forming a first group of said received slots at least equalin number to said known minimum length and a counterpart group of thesame number of received slots but spaced from said first group by saidknown interval, each group having a first slot and a last slot;

[0009] (iii) for each slot in said first group and its correspondingslot in the counterpart group, performing a correlation operation toobtain a correlation value for information in each of thesecorresponding pairs of slots;

[0010] (iv) summing said obtained correlation values to obtain acorrelation sum and storing said obtained correlation sum;

[0011] (v) obtaining a next correlation sum by:

[0012] (a) performing a correlation operation to obtain a correlationvalue for information in the slot at the first slot of said first groupand the corresponding slot in said counterpart group;

[0013] (b) performing a correlation operation to obtain a correlationvalue for information in the slot outside the first group adjacent tothe last slot of said group and the corresponding slot outside said lastslot of said counterpart group;

[0014] (c) reforming said first group and said counterpart group toexclude the respective slots first slots and to include the respectiveslots adjacent the last slots; and

[0015] (d) from the correlation sum last obtained, subtracting thecorrelation value obtained in (a) and adding the correlation valueobtained in (b) to obtain a correlation sum;

[0016] (vi) storing said correlation sum obtained in (d);

[0017] (vii) repeating steps (v) and (vi) until a number of correlationsums equal to said known minimum length are obtained and stored;

[0018] (viii) examining said stored obtained correlation sums to selectthe sum with the greatest magnitude, this selected sum indicative of thepresence of said sequence and thereby indicating the slot timing.

[0019] According to another aspect of the present invention, there isprovided a method of determining frequency offset in a radio receiverfrom a signal transmitted by a radio transmitter which transmits, in atime-slotted arrangement, at least one signal which includes a sequenceof information indicating the slot timing and having a length at leastequal to a known minimum length and which sequence is repeated in saidat least one signal at a known interval, comprising the steps of:

[0020] (a) receiving said signal and determining the slot timing of saidsignal;

[0021] (b) forming a vector comprising determined correlation valuesbetween an instance of said minimum length of said known information asreceived by said receiver and said known information signal;

[0022] (c) repeating step (b) for additional received instances of saidminimum length of said known information to obtain a set of vectors; (d)forming an inner product of said set of vectors to obtain a set ofobtained products;

[0023] (e) determining an average value of said obtained products anddetermining the arctangent of the average value;

[0024] (f) determining from the nominal frequency of interest and apredetermined maximum error in the receiver the values, if any, whichcan be added to the determined arctangent;

[0025] (g) from${{\Delta \quad f} = \frac{B}{2{\pi ({interval})}T}},$

[0026] where B=tan⁻¹ (average value)+2π (the values determined in step(f)) and interval is the number of slots between the start of instancesof the signal, determining the possible frequency offsets Δf for eachvalue determined in step (f) and testing each determined value byapplying it to said minimum length of said known information as receivedby said receiver and then correlating the resulting information withsaid known information signal; and

[0027] (h) selecting the possible frequency offset with the bestcorrelation value determined in step (g) as the frequency offset.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Preferred embodiments of the present invention will now bedescribed, by way of example only, with reference to the attachedFigures, wherein:

[0029]FIG. 1 shows the radio channel frame and slot structure proposedby the 3GPP organization and the arrangement of a synchronization signaltherein;

[0030]FIG. 2 shows a graphical representation of a correlation operationin a conventional acquisition method;

[0031]FIG. 3 shows a graphical representation of a correlation operationin accordance with the present invention; and

[0032]FIG. 4 shows a plot of the absolute value of a correlation valueversus the received chip at which the correlation operation wasperformed.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The 3GPP system, discussed above, includes a primarysynchronization channel (PSCH) and a secondary synchronization channel(SSCH) which are broadcast from each base station and which are used byeach customer device to acquire the base station. In the 3GPP system,all channels (including these two synchronization channels) arebroadcast in the form of slotted frames, with most channels havingframes of ten milliseconds (10 ms) duration and wherein each frameincludes fifteen slots.

[0034] Presently, the 3GPP system is generally contemplated as beingbased upon CDMA multiplexing techniques and the following discussionrelates to a CDMA related embodiment of the present invention. However,as will be apparent to those of skill in the art, the present inventioncan be applied to other multiplexing techniques including OFDM, FDMA,TDMA and combinations of such techniques such as GSM.

[0035] As discussed in the 3GPP documentation, the PSCH is used bycustomer devices to determine the timing of the slots within framestransmitted by a base station. A predefined data sequence, the primarysynchronization sequence, is transmitted in the slots and frames of thePSCH and this sequence has been defined and arranged such that customerdevices can determine the start time of the slots in the framestransmitted from the base station by determining the location of thissequence in a received set of chips (in a CDMA implementation).

[0036] Once the slot timing has been determined by a customer devicefrom the PSCH, the SSCH is examined by the customer device to determinethe timing of the frames, i.e. the start time of each frame, and otherinformation, including scrambling codes used by the base station, etc.The acquisition and processing of the PSCH and SSCH channels isperformed at the start up of a customer device as it first acquires abase station and, in mobile systems at least, is performed on an ongoingbasis for adjacent cells to permit handoffs between cells as thecustomer device moves between service areas.

[0037]FIG. 1 shows a PSCH frame, from the 3GPP system. As shown, a frame20 includes fifteen slots 24, through 2425. Each slot 24 i includestwo-thousand, five-hundred and sixty (2,560) chips so that frame 20 hasa total of thirty-eight thousand, four hundred (38,400) chips. Thebroadcast duration of frame 20 is ten milliseconds, for a chip rate ofthree million, eight-hundred and forty thousand (3.84 million) chips persecond.

[0038] As indicated, the primary synchronization sequence 28 isbroadcast in the first two hundred and fifty six chips of each slot 24i. As part of the acquisition of the signals transmitted by a basestation, the customer device performs a correlation between the receivedsignal and the known two-hundred and fifty-six predefined chips ofprimary synchronization data to determine the slot timing.

[0039] Conventionally, as shown in FIG. 2, this correlation is performedat every chip c_(i) in a block of received data 32 and requires chipc_(i) and the two-hundred and fifty-five following received chips (chipsc_(i+1) to c_(i+255)) to be correlated with the two-hundred andfifty-six known values for the primary synchronization-sequence 28 andthe results summed and compared to those obtained when starting at eachother chip c_(i). In fact, to ensure identification of the primarysynchronization sequence 28 with a high degree of confidence, thecorrelation operation is typically performed starting at each of chipsc₁ to c₂₅₆₀, thus requiring the processing of two-thousand eight-hundredand sixteen chips (2,816), which is equivalent to one and a tenth slotsof received data.

[0040] In FIG. 2, the first correlation operation (shown at the top ofthe Figure) is performed starting at chip c₁ and proceeding through chipC₂₅₆ with the two-hundred and fifty-six values of the primarysynchronization sequence 28. The second correlation operation (shownbelow the first) starts at chip c₂ and proceeds through chip c₂₅₇ withthe same two-hundred and fifty-six values of the primary synchronizationsequence 28. The correlation operation is performed for each subsequentstarting chip c_(i), up to the last operation, shown at the bottom ofthe Figure, which is performed starting on chip c₂₅₆₀ and proceedingthrough chip c₂₈₁₆ with the two-hundred and fifty-six values of theprimary synchronization sequence 28.

[0041] The highest absolute valued result of these operations,indicating the best correlation, should be found at the location of thefirst received chip of the primary synchronization sequence 28, thusindicating the start timing of the received slots.

[0042] When actually implemented, the received chips are typically atleast double-sampled and filtered with a square root raised cosinefilter. Thus, the known copy of the primary synchronization sequence isalso at least double-sampled and filtered with a square root raisedcosine filter before the correlation is performed.

[0043] While this conventional technique does work, it requires a largeamount of computation to be performed. Specifically, performing thecorrelation for each one and a tenth slots requires six-hundred andfifty-five thousand, three-hundred and sixty (655,360) complexmultiplication operations (256 multiplications of complex numbers foreach of 2816 chips), multiplied by the sampling rate, which is typicallyat least two, and the same number of addition operations. Thus, thistechnique can be both computationally expensive and time consuming.

[0044] The above-mentioned disadvantage of the conventional techniquecan be further exacerbated if other signals broadcast by the transmitterbeing acquired are not well behaved. Specifically, in the 3GPP system,all data is transmitted based upon the ten millisecond frame, fifteenslot, structures discussed above. Thus, the customer device isperforming the correlation on the total received signal which caninclude the signals transmitted on other channels. In the 3GPP system,only QPSK (quadrature phase shift keying) modulation is employed fortransmitted signals and the power level of signals modulated with QPSKis not excessive with respect to the power levels of the PSCH and SSCHsignals. In such circumstances, acquisition can often be achieved aftercorrelating one or a few slots of received data.

[0045] However, if other signals are transmitted using QAM (quadratureamplitude modulation) modulation, as proposed by the assignee of thepresent invention, or other modulation techniques, or the signals areotherwise not well behaved, the power levels of transmitted signals onsome channels relative to those of the PSCH and SSCH may be large,requiring correlation to be performed over one or more frames ofreceived signal before sufficient confidence is obtained in the result.Correlating a single frame of received signal with the above-describedconventional acquisition technique requires over nine million complexnumber multiplication operations to be performed, as well as requiringlarge amounts of memory to store the correlation results for comparison.

[0046] The present inventor has determined that slot timing can beobtained in a more computationally, time and memory-efficient manner,relative to the conventional acquisition technique, according to thefollowing system and method.

[0047] Unlike the conventional method discussed above, wherein the knownprimary synchronization sequence is correlated over at least an entireslot of the received signal, the present invention tales advantage ofthe fact that the primary synchronization sequence repeats after aknown, and fixed, number of chips. In the 3GPP system, the primarysynchronization sequence is transmitted in the first two-hundred andfifty-six chips of the two-thousand, five-hundred and sixty chips of aslot and the primary synchronization sequence is repeated every slot.Accordingly, the present invention determines the correlation between areceived chip and a counterpart chip received two-thousand, five-hundredand sixty chips (i.e.—one slot) before or after. Thus, the correlationvalue, Cor(k), at a received chip, r(k), is the sum of the two-hundredand fifty-six complex multiplications (multiplications of complexnumbers) of each of the received chips r(k) to r(k+256) with receivedcounterpart chips r(k+2560) to r(k+2560+256) or${{Cor}(k)} = {\sum\limits_{i = k}^{k + 256}{{r(i)}*{{r\left( {i + 2560} \right)}.}}}$

[0048] For the first iteration, Cor(k) is solved for k, where k is thefirst received chip to be examined, and this requires two hundred andfifty six complex multiplication operations and the same number ofaddition operations to be performed.

[0049] Next, Cor(k+z), where z varies over the balance of a completeslot, i.e. 1<z<2559, must be determined. However, as will be apparent,by correlating the received signal with an offset counterpart receivedsignal, a moving sum method can be employed. Unlike the conventionalmethod wherein two-hundred and fifty-six multiplication and additionoperations are required to be performed when the correlation is to beperformed for the next received chip, with the moving sum method of thepresent invention, Cor(k+z) is determined from

Cor(k+z)=Cor(k+z−1)−(r(k+z−1)*r(k+z−1+2560))+(r(k+256+z)*r(k+256+z+2560))

[0050] In other words, after any Cor(k+z−1) has been determined (and onthe very first iteration z=1 and Cor(k+z−1)=Cor(k)), the nextcorrelation value Cor(k+z) can be obtained by subtracting from the valuedetermined for Cor(k+z−1) the correlation value of the first receivedchip r(k+z−1) in that last determined value with its offset receivedchip r(k+z−1+2560) and determining and adding a correlation value forthe newly included received chip r(k+z+256) and its offset received chipr(k+2560+z+256).

[0051] Thus, the first determined correlation value Cor(k) requirestwo-hundred and fifty-six complex multiplication and addition operationsand each subsequent Cor(k+z) requires two additional complexmultiplications [one to calculate r(k+z−1)*r(k+z-1+2560) to besubtracted and one to calculate r(k+z+256)*r(1+2560+z+256) to be added],and one addition and one subtraction operation.

[0052]FIG. 3 illustrates this graphically. In the Figure, a number ofreceived chips 40 has been obtained. The repeated sequence 42, has alength of two-hundred and fifty-six chips and is repeated everytwo-thousand and fifty-six chips.

[0053] A first received chip r₁, is selected arbitrarily, and all therest of the received chips r₁ are considered relative to this arbitrarystarting point. In other words, received chip r₂₅₆ is defined as havingbeen received two-hundred and fifty-five chips after received chip r₁and received chip r₂₅₆₂ is defined as having been received two-thousand,five-hundred and sixty-one chips after received chip r₁. In the Figure,one instance of repeated sequence 42 is shown commencing at receivedchip r₁₀₂₀ and the next instance is shown commencing at received chipr₃₅₈₀.

[0054] Received chips r₁ through r₂₅₆ are correlated against receivedchips r₂₅₆₁ through r₂₈₁₆, respectively, to solve for Cor(1), with r₁being correlated with r₂₅₆₁, r₂ being correlated with r₂₅₆₂, etc. NextCor(2) is solved for by taking the value determined for Cor(1) andsubtracting the contribution 44 of the correlation of r₁ and r₂₅₆₁, andadding the contribution 48 of the correlation of r₂₅₇ and r₂₈₁₇.Typically, the process is repeated for at least the known interval overwhich the signal is repeated, such as a slot. In the present example,wherein the primary synchronization sequence repeats every two-thousand,five-hundred and sixty chips, at least Cor(1) through Cor(2560) aredetermined. FIG. 4 shows the result of the absolute value of Cor(1)through Cor(2560). As can been seen, the peak value occurs for Cor(1020)which is a start location of the repeated sequence.

[0055] Even if more than one slot length of signal must be processed,for sufficient confidence, the method merely continues subtracting andadding contributions from each previous and successive pair of chips.The present inventors refer to this as a fast scan acquisition method.

[0056] Thus, determining correlation values over an entire slot with thefast scan acquisition method requires much less computation that theconventional acquisition method described above, and this difference iseven greater if more than one slot must be processed. Further, while thediscussion above has been with reference to the presently proposed 3GPPsystem wherein a predefined synchronization sequence is employed, thefast scan acquisition method can also be employed in systems wherein thesynchronization sequence is not predefined (or known) to the customerdevice. Specifically, as long as the interval at which thesynchronization sequence is repeated and the sequence has at least aknown minimum length, the fast scan acquisition method can be employed.

[0057] Further, the sequence need not comprise a contiguous set of chips(i.e.—some number of adjacent chips) nor is the sequence limited to anyparticular number of chips (i.e.—two-hundred and fifty-six chips vs.three-hundred chips). The sequence can comprise any periodic sequence ofany desired length, as will be apparent to those of skill in the art,where the periodicity of the sequence and a minimum sequence length isknown by the receiver. For example, the synchronization sequence cancomprise: chips one through seventy-five; chips one-thousand throughone-thousand two-hundred; and chips two-thousand through two-thousandand seventy-five of a slot. The length will be selected to provide adesired level of confidence in the result, with longer sequencesgenerally providing greater levels of confidence.

[0058] It is contemplated that less than a full sequence can beprocessed by a particular receiver. For example, the sequence can occupythe entire slot and one receiver may only consider one-tenth of the slotwhile another, which requires a greater degree of confidence in theresult, can process one-half or even all of the sequence.

[0059] Many other sequences of suitable lengths and periodicities willbe apparent to those of skill in the art and are limited only by thewell known design requirements for such sequences, including the needfor the sequence to generate an appropriate autocorrelation response andto have a sequence length long enough to provide sufficient confidencein the correlation result.

[0060] Depending upon operating conditions experienced by a customerdevice, and in particular for low received SNR's, the fast scanacquisition method disclosed above may not afford sufficient confidenceand/or accuracy in the result. In such a case, the fast scan acquisitionmethod above can be used to determine a best estimate of the location ofthe start of the slots, k., by identifying the peak absolute Cor( )value from the set of determined Cor(k)'s, i.e.—from

k _(est) =arg max (|Cor(k)|), 1≦k≦2560

[0061] (the upper limit of k can exceed 2560 if more than one receivedslot was processed). Once knit has been determined, conventional-typeacquisition methods, as described above with respect to FIG. 2 or anyother suitable technique, can be employed over a selected number ofreceived chips preceding and following k_(est) refine the slot timing.For example, the region of interest can be deemed to extend fromk_(est)−40 to k_(est)+40 and the conventional acquisition techniquesdescribed above can be used on this interval for refinement.

[0062] If additional accuracy is required, perhaps due to very low SNRlevels, etc., both the above-described fast scan and/or fast scan andconventional acquisition over a selected region of interest, can beperformed independently in the I and Q quadratures. Further, these stepscan be performed over several slots, an entire frame or even multipleframes until a desired level of accuracy and/or confidence is obtained.

[0063] An additional problem in acquiring a transmitter occurs indetermining the frequency offset which is experienced at the receiverdue to oscillator error at the receiver. For example, achievingoscillator accuracy of greater than three parts per million (3 ppm) isexpensive and many systems, for cost reasons, specify that a 5 ppmoscillator is sufficient. However, the receiver must be able todetermine the frequency offset which results from this oscillator errorin order to correctly receive signals.

[0064] If a transmitter transmits a signal S_(k), then in a sampleddomain the receiver will receive

r(k)=ŝ(k)e ^(jΔf2πkt) e ^(Jφ)+(k)

[0065] where ŝ is the received version of the transmitted signal(resulting from multi-path effects), Δf is the frequency offset, 4 isthe initial phase, T is the duration of a chip and n is the noise. In acontinuous domain, the received signal is

r(t)=ŝ(t)e ^(j2πkt) e ^(jφ) n(t)

[0066] In the present invention, after slot timing has been determined,a vector V is constructed of the first two hundred and fifty six chipsof each slot in a received frame which are correlated with the knownprimary synchronization sequence (psc_(i)) to obtain vectors${{{V(1)} = \begin{pmatrix}{{psc}_{1}*r_{1}} \\\vdots \\{{psc}_{256}*r_{256}}\end{pmatrix}},{{V(2)} = \begin{pmatrix}{{psc}_{1}*r_{2560 + 1}} \\\vdots \\{{psc}_{256}*r_{2560 + 256}}\end{pmatrix}},\ldots \quad,{{V\left( N_{acc} \right)} = \begin{pmatrix}{{psc}_{1}*r_{{({14*2560})} + 1}} \\\vdots \\{{psc}_{256}*r_{{({14 + 2560})} + 256}}\end{pmatrix}}}$

[0067] As indicated above, it may be desired and/or required to considermore than a frame of slots of received signal to achieve the desiredlevel of confidence and/or accuracy. Thus, N_(acc) slots can beconsidered, where N_(acc) can be greater than fifteen (or in other slotstructures, less than fifteen). The frequency offset information iscontained in vectors Vik). One method to extract this information isdescribed below, but other suitable methods will be apparent to those ofskill in the art.

[0068] To obtain the frequency offset information, the inner product ofV can be determined to yield N_(acc)−1 data points

α(m)=V(m)^(·T) V(m+1), m=1, 2, . . . , (N _(occ)−1)

[0069] which are proportional to e^(j2π2560ΔfT). Then, defining${\frac{1}{N_{acc}}{\sum\limits_{m = 1}^{N_{acc}}{\alpha (m)}}} = A$

[0070] and

2π2560ΔfT=∠(A)=B

[0071] everything needed to determine Δf is known, except B=2n2560Δfmodulo 2π. However, as mentioned before, the oscillator in a receiver istypically specified as having a known maximum error and the range ofpossible values for B can easily be determined. For example, if thereceiver oscillator is specified as having a maximum error of 5 ppm andif the transmission frequency is 1.8 GHz, then B can only have thirteenvalues, specifically B=∠A+2π×{0, ±1, ±2, . . . , ±6}. As will beapparent to those of skill in the art, if the primary synchronizationsequence is a different number of chips and/or the number of chips perslot and/or the maximum oscillator error differ in otherimplementations, the above operations will be modified appropriately.

[0072] Therefore, each of these thirteen possible values of B isevaluated by applying the value B_(i) to the received primarysynchronization sequence 28 (in our example the first two-hundred andfifty-six chips in a received slot) and correlating the result to theknown primary synchronization sequence 28. The best correlation willoccur with the correct value for B. Once B is known, the frequencyoffset, Δf, can be derived from${\Delta \quad f} = \frac{B}{2\quad \pi \quad 2560T}$

[0073] as all of the other quantities are now known.

[0074] If it is desired to determine the initial phase φ, to initializea RAKE receiver for example, this can now also easily be determined. Thevectors V( ), described above, can also be used for this purpose. Ifκ=Σe^(jkThf2π), then φ can be determined from$^{j\varphi} = {\sum\limits_{k = 1}^{N_{acc}}\frac{{sum}\left( {V(k)} \right)}{N_{acc}*\kappa}}$

[0075] As discussed above, the present invention can provide significantadvantages over prior art acquisition methods and systems by reducingcomputational complexity, memory requirements and the time required toacquire a radio transmitter. In addition, the method and system of thepresent invention can also be employed in other advantageous manners.

[0076] As an example of one such additional use of the presentinvention, if a customer device employs a steerable antenna (eitherelectrically or mechanically steerable) to receive signals from a basestation transmitter, the fast scan acquisition method described abovecan be employed to quickly determine an antenna direction withacceptable reception characteristics. For example, if an electricallysteerable antenna with four possible directions is employed, eachdirection can be selected in turn and the fast scan acquisition methodcan be performed for that direction and the results of the fast scanfrom each direction can be used to select an acceptable direction forfurther communications. In an embodiment of the present invention, themagnitudes of the peak correlation value determined for each directionare compared and the greatest magnitude direction is selected.

[0077] Another example of an additional use of the present invention isfor a customer device to monitor reception levels of other basestations, or base station sectors (in the case of multi-sector basestations) to permit handoff of the customer device between base stationsor sectors. In this context, the customer device can, on an intermittentbasis, perform a fast scan for each base station, or base stationsector, of interest to obtain an initial indication of the receptionlevels at the customer device for each transmitter. The customer devicecan use this information to request hand-off from a present base stationor base station sector to another base station or base station sectorwhich it can receive at better levels, or this information can betransmitted to the base station from the customer device, and then on toa network management system which can monitor and/or determine if ahandoff should be performed.

[0078] Other uses and advantages of the present invention will beapparent to those of skill in the art. The above-described embodimentsof the invention are intended to be examples of the present inventionand alterations and modifications may be effected thereto, by those ofskill in the art, without departing from the scope of the inventionwhich is defined solely by the claims appended hereto.

We claim:
 1. A method for a radio receiver to acquire timing informationfor a radio transmitter which transmits, in a time-slotted arrangement,at least one signal which includes a sequence of information indicatingthe slot timing and having a length at least equal to a known minimumlength and which sequence is repeated in said at least one signal at aknown interval, the method comprising the steps of: (i) receiving anumber of slots of said at least one signal which is at least sufficientto allow reception of two repetitions of said known minimum length ofsaid sequence; (ii) forming a first group of said received slots atleast equal in number to said known minimum length and a counterpartgroup of the same number of received slots but spaced from said firstgroup by said known interval, each group having a first slot and a lastslot; (iii) for each slot in said first group and its corresponding slotin the counterpart group, performing a correlation operation to obtain acorrelation value for information in each of these corresponding pairsof slots; (iv) summing said obtained correlation values to obtain acorrelation sum and storing said obtained correlation sum; (v) obtaininga next correlation sum by: (a) performing a correlation operation toobtain a correlation value for information in the slot at the first slotof said first group and the corresponding slot in said counterpartgroup; (b) performing a correlation operation to obtain a correlationvalue for information in the slot outside the first group adjacent tothe last slot of said group and the corresponding slot outside said lastslot of said counterpart group; (c) reforming said first group and saidcounterpart group to exclude the respective slots first slots and toinclude the respective slots adjacent the last slots; and (d) from thecorrelation sum last obtained, subtracting the correlation valueobtained in (a) and adding the correlation value obtained in (b) toobtain a correlation sum; (vi) storing said correlation sum obtained in(d); (vii) repeating steps (v) and (vi) until at least a number ofcorrelation sums equal to said known minimum length are obtained andstored; (viii) examining said stored obtained correlation sums to selectthe sum with the greatest magnitude, this selected sum indicative of thepresence of said sequence and thereby indicating the slot timing.
 2. Themethod of claim 1 wherein said indication of slot timing in step (viii)is employed to identify a region of interest in said received number ofslots of said at least one signal for processing by a subsequentacquisition operation.
 3. The method of claim 1 wherein the sequence ofinformation indicating the slot timing is known to both the transmitterand receiver and, once said slot timing is determined, the frequencyoffset in said received is determined from the steps of: (1) forming avector comprising determined correlation values between said an instanceof said minimum length of said known information as received by saidreceiver and said known information signal; (2) repeating step (1) foradditional received instances of said minimum length of said knowninformation to obtain a set of vectors; (3) forming an inner product ofsaid set of vectors to obtain a set of obtained products; (4)determining an average value of said obtained products and determiningthe arctangent of the average value; (5) determining from the nominalfrequency of interest and a predetermined maximum error in the receiverthe values, if any, which can be added to the determined arctangent; (6)from ${{\Delta \quad f} = \frac{B}{2{\pi ({interval})}T}},$

where B=tan⁻¹(average value)+2π (the values determined in step (5)) andinterval is the number of slots between the start of instances of thesignal, determining the possible frequency offsets Δf for each valuedetermined in step (5) and testing each determined value by applying itto said minimum length of said known information as received by saidreceiver and then correlating the resulting information with said knowninformation signal; and (7) selecting the possible frequency offset withthe best correlation value determined in step (6) as the frequencyoffset.
 4. The method of claim 1 wherein the sequence of informationindicating the slot timing is known to both the transmitter and receiverand, once said slot timing is determined, the initial phase offset atsaid receiver is determined from the steps of: (1) forming a vector Vcomprising determined correlation values between said instance of saidminimum length k of said known information as received by said receiverand said known information signal for N_(acc) instances of said receivedinformation; (2) determining the initial phase ¢ from$^{j\varphi} = {\sum\limits_{k = 1}^{N_{acc}}\frac{{sum}\left( {V(k)} \right)}{N_{acc}*\kappa}}$

where T represents the duration of each signal and Δf represents thefrequency offset.
 5. A method of determining frequency offset in a radioreceiver from a signal transmitted by a radio transmitter whichtransmits, in a time-slotted arrangement, at least one signal whichincludes a sequence of information indicating the slot timing and havinga length at least equal to a known minimum length and which sequence isrepeated in said at least one signal at a known interval, comprising thesteps of: (a) receiving said signal and determining the slot timing ofsaid signal; (b) forming a vector comprising determined correlationvalues between an instance of said minimum length of said knowninformation as received by said receiver and said known informationsignal; (c) repeating step (b) for additional received instances of saidminimum length of said known information to obtain a set of vectors; (d)forming an inner product of said set of vectors to obtain a set ofobtained products; (e) determining an average value of said obtainedproducts and determining the arctangent of the average value; (f)determining from the nominal frequency of interest and a predeterminedmaximum error in the receiver the values, if any, which can be added tothe determined arctangent; (g) from${{\Delta \quad f} = \frac{B}{2{\pi ({interval})}T}},$

where B=tan⁻¹(average value)+2π(the values determined in step (f)) andinterval is the number of slots between the start of instances of thesignal, determining the possible frequency offsets Af for each valuedetermined in step (f) and testing each determined value by applying itto said minimum length of said known information as received by saidreceiver and then correlating the resulting information with said knowninformation signal; and (h) selecting the possible frequency offset withthe best correlation value determined in step (g) as the frequencyoffset.